In many optical local area networks (LANs) and optical backplanes that have a bus-type architecture, an optical repeater receives and combines optical signals form optical transmitters in system nodes, as well as amplifies and individually retransmits signals to optical receivers in the system nodes. FIG. 1 illustrates such a prior art optical transmission system. Nodes 106 through 110 transmit and receive optical signals to and from optical repeater 100 by way of optical fibers 130 through 139. Specifically, optical repeater 100 receives and combines optical signals via optical fibers 135 through 139 and redistributes these optical signals via optical fibers 130 through 134. Optical combiner 115 receives and combines the optical signals; whereupon components 116, 117, and 118 electrically process the combined signal. Optical unit 123 then converts the combined electrical signals to an optical signal and transfers it to optical splitter 124 via optical link 140 which communicates the optical signal on links 130 through 134. Nodes 106 through 110 are identical with each having a controller, receiver unit, transfer unit, and power control as illustrated for node 106. Optical combiners and splitters, such as units 115 and 116, are generically refereed to herein as optical couplers.
FIG. 2 illustrates an optical coupler in accordance with U.S. Pat. No. 4,913,508 which performs the functions of couplers 115 and 124. The optical signals from optical fiber bundle 201 are coupled via optical coupler 203 to optical fiber 204. Similarly, an optical signal from optical fiber 204 may be coupled to optical fiber bundle 201. The cavity of optical coupler 203 forms the optical core of the optical coupler and is filled with a material that creates a waveguide with substantially the same numerical aperture as optical fiber 204 and optical fiber bundle 201. By matching the numerical apertures, the efficient transfer of optical energy is achieved between the optical fibers in spite of possible refractive index mismatch between the optical core and optical fiber bundle 201 and optical fiber 204.
One of the problems of making optical couplers of the type illustrated in FIG. 2 is the packing density achieved using round fibers in optical fiber bundle 201. The theoretical efficiency for perfectly uniform illumination of optical fiber bundle 201 by optical fiber 204 is given by the total core area of optical fiber bundle 201 divided by the total cross-sectional area of the cavity of optical coupler 203. In general, the theoretical efficiency for perfectly uniform illumination is limited to the range of 50-60% for various numbers of fibers using the optical coupler illustrated in FIG. 2.
The disadvantage of utilizing round optical fibers in a round cavity is illustrated in FIG. 3 and 4. FIG. 3 illustrates the case where the diameter of the cavity of the optical coupler is equal to four times the diameter of optical fiber 204. FIG. 4 illustrates the case where the diameter of the cavity of the optical coupler i equal to eight times the diameter of optical fiber 204. As illustrated, the optical coupler of FIG. 3 has a theoretical efficiency of 49%; and the optical coupler of FIG. 4 has a theoretical efficiency of 59%. Further, for optical fibers having polymeric cladding and cores, the cladding cross-sectional area is extremely small compared to the interstitial space between bundled optical fibers. Thus the removal of the cladding is costly and difficult with only a tiny increase in efficiency that might be gained. In the coupler illustrate din FIG. 2, the numerical apertures of the fibers and the polymeric mixing region are closely matched and the reflection is extremely small. Hence, the inefficiency caused by the interstitial space between round fibers is the only significant source of inefficiencies in this type of coupler.
It is known in the art to mil optical glass preforms into D-shaped cross-sections and to draw D-shaped fibers from these preforms. These D-shaped optical fibers are then put together in circular cross-sections to make up 2.times.2 optical couplers. However, this method has the disadvantage of extremely high milling cots and material waste, since the entire optical fiber has to be in the D-shape. Similarly, other fiber shapes can be made by first milling an optical preform to form optical fibers but would suffer from the same disadvantages as the use of this method to form D-shaped optical fibers.